Issue with 525.2 surrogates

I've been doing 525.2 for close to 16 years, and lately been having issues with perylene d 12, and triphenyl phosphate failing the +/- 30% of targeted concentration. I don't think the issue is due to my sample concentration step, as the nitro xylene recovers very well. The triphenyl phosphate sometimes recover over 130% and the perylene d 12< 70%.

With what I see it may be due to the way I handle my surrogate mix. The mix is a commercial 3 component mix @ 500ppm in acetone.Manufacturer's storage requirements are ambient temp. I store it in a freezer that is usually about -15C. Prior to use I warm it and then vortex.

My theory is that the perylene d12 falls out of solution in the freezer and warming and mixing is not enough. the other week I prepared a continung calibration std. using the mix of compounds used to spike my LFM/Bs. I combined this mix, and the IS and surrogates and brought to volume. When I ran it against the initial calibration everything was very much in range with the exception of the perylene d12, which recovered in the mid 70%. I see similar results using the con cal prepared with the initial calibration.

The implication is that I am not completely reconstituting the perylene d12.

Any thoughts?

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